The invention relates to a data transmission method used in a CDMA-type radio system comprising at least one base station and a plurality of terminal equipments that exchange data at least in a packet switched mode, a terminal equipment transmitting to the base station on a random access channel a random access signal comprising at least a preamble and a data part multiplied by a spreading code.
The invention also relates to a radio system that is of the CDMA type and that comprises at least one base station and a plurality of terminal equipments which are arranged to exchange data at least in a packet switched mode, a terminal equipment being arranged to transmit to the base station on a random access channel a random access signal that comprises at least a preamble and a data part multiplied by a spreading code.
At present, packet switched networks are a particular object of interest in further development of code division multiple access (CDMA) and wideband CDMA (W-CDMA) systems. Especially third-generation mobile systems, such as the universal mobile telephone system (UMTS), are being provided with arrangements enabling packet transmission.
Packet switching is a method where a connection is set up between users by transferring data in packets that contain address and control data in addition to the actual data. Several connections may employ the same transmission channel simultaneously. The packet switching method is suitable for data transmission where the data to be transmitted is generated in bursts. In such a case, it is not necessary to allocate a data link for the entire duration of transmission but only for the time it take to transmit the packets. This reduces costs and saves capacity considerably during both the set-up and use of the network. In order for data transmission to be sufficiently fast, applications utilizing packet switching require high reliability of transmission for example in order to avoid retransmissions.
The problem with packet switched transmission in the uplink direction in CDMA systems is how to implement sufficiently fast synchronization for bursts while keeping the interference caused by the transmission to other users as small as possible. Fast synchronization is only possible if the transmission power of the bursts is high enough. The transmission power in turn affects directly the interference caused to other users. In prior art arrangements, there have been efforts to solve these problems such that a terminal equipment keeps transmitting a preamble of a random access signal with a gradually increasing power until the base station sends an acknowledgement of synchronization with the signal transmitted by the terminal equipment. The terminal equipment then fixes the power of the random access signal at the level it had during the acknowledgement. This arrangement has an advantage that the transmission power is as low as possible. A drawback is that the synchronization is slow, wherefore the total transfer rate remains small. In another prior art arrangement the aim is to immediately determine the correct transmission power for a burst by estimating the transmission power by means of open loop power control. This takes place by utilizing measurements in the downlink direction and the data transmitted by the base station about for example the interference level of the combined signal it has received. In order to reduce the likelihood of simultaneous transmissions with the same code, the terminal equipment selects the moment of burst transmission and the spreading code it uses randomly from a set of alternatives. A New Slotted ALOHA Based Random Access Method for CDMA Systems by R. Esmailzadeh, M. Gustafsson (IEEE Proceedings, International Conference on Universal Personal Communications, October 1997), which is incorporated herein by reference, discloses an arrangement for improving data transfer rate in packet switched communication. The method comprises two steps: the terminal equipment transmits a packet switched random access signal for call set-up in a certain time slot, and the base station correspondingly accepts the request of the random access signal if the signal received by the base station is error-free. Before the terminal equipment transmits the random access signal, it estimates the transmission power required. However, this arrangement does not solve the problem of interference caused to other users. Also, the terminal equipment must estimate the transmission power in the downlink direction, even though it transmits itself in the uplink direction where the radio channel is usually totally different and therefore the risk of selecting a wrong transmission power is great.
The object of the invention is to realize a method and a radio system implementing the method which solve the aforementioned problems while providing rapid synchronization, decreasing the amount of interference and increasing the data transmission capacity.
This is achieved with a data transmission method of the type described in the preamble that is characterized in that a predetermined set of spreading codes and signature sequences are stored at the terminal equipment or the equipment generates such a predetermined set of spreading codes and signature sequences, and each signature sequence determines one spreading code; the terminal equipment selects one signature sequence by a random process from the set of signature sequences and adds the selected signature sequence to the preamble of the random access signal; the terminal equipment uses the spreading code corresponding to the selected signature sequence in the data part of the random access signal; interference cancellation is performed at the base station according to the signature sequence of the preamble of the received random access signal, such that at least the interference caused by the received data part is eliminated from at least one other received signal in order to increase data transmission capacity and to improve detection.
The radio system according to the invention is also characterized in that the terminal equipment comprises signature sequence means and spreading code means that are arranged to provide a predetermined set of spreading codes and signature sequences, each signature sequence determining one spreading code; the terminal equipment is arranged to select one signature sequence by a random process from the set of signature sequences, and to add the selected signature sequence to the preamble of the random access signal; the terminal equipment is arranged to use the spreading code corresponding to the selected signature sequence in the data part of the random access signal; the base station is arranged to perform interference cancellation according to the signature sequence of the preamble of the received random access signal, such that the base station is arranged to eliminate at least the interference caused by the received data part from at least one other received signal in order to increase transmission capacity and to improve detection.
The data transmission method and the radio system according to the invention provide several advantages. The arrangement according to the invention provides faster synchronization and a greater data transfer rate for each packet switched channel than the prior art solutions, without impairing the signal-to-interference ratio of the other channels.